The invention is directed to a method for level limitation of a time-discrete signal in a digital hearing aid comprising at least one input transducer, an A/D converter, a signal processing unit and an output transducer. The invention is also directed to a digital hearing aid for the implementation of the method.
A hearing aid user whose hearing is supplied with unusual sound events via a hearing aid can find this unpleasant, even painful. Such an unusual sound event can, for example, be the noise of a motor that begins suddenly. For limiting the output acoustic pressure, it is therefore necessary to limit the output level. For this purpose, a discomfort threshold is determined when fitting the hearing aid. The hearing aid is then set such that an adjustable, maximum output acoustic pressure is not exceeded even given extremely loud input signals. The hearing aid comprises a unit for level limitation (peak clipping) for this purpose. Such a hearing aid is disclosed, for example, by DE 41 25 379 C1. The system for level limitation deriving therefrom are implemented in analog circuit technology.
DE 197 28 482 C1 discloses a circuit for dividing a signal into a plurality of sub-signals, whereby core filters having double the sampling rate are employed in order to achieve an identical edge steepness to preceding stages.
EP 0 656 737 B1 discloses a hearing aid with digital signal processing. Therein, a digital unit for level limitation are provided between the A/D converter and the D/A converter. The values of a digital signal are thereby set to an adjustable maximum value before the D/A conversion. This results in distortions of the signal occurring in the following D/A conversion and non-harmonic signal parts arising in the audible frequency range.
An object of the invention is to specify a method for improving the sound quality in a digital hearing aid unit with digital level limitation. A further object of the invention is to design a digital hearing aid unit with a digital level limitation such that an improvement of the sound quality can be achieved.
According to the method and apparatus of the invention for a level limitation of a time-discrete signal in a digital hearing aid, at least one input transducer, an A/D converter in which an input signal is sampled with a specific sampling rate, a signal processing unit, and an output transducer are provided. The sampling rate of the time-discrete signal is boosted in a signal path following the A/D converter. Signal values of the time-discrete signal with the boosted sampling rate are limited to an adjustable maximum value.
When a signal is limited strictly to a maximum value, i.e. all signal values that exceed this maximum value are equated with the maximum value, then the Fourier analysis of the resulting signal shows that the fundamental is attenuated and harmonica are attached. For example, the following applies to the signal fxe2x80x2(t) that proceeds from the analog signal f(t)=sin xcfx890t by clipping at +/xe2x88x920.5:
fxe2x80x2(t)=0.61 sin xcfx890t+0.14 sin 3xcfx890t+0.02 sin 5xcfx890t+ . . . 
The spectrum of the analog signal periodically repeats with the sampling frequency given the presentation of the digitalized signal in the frequency domain. As a result thereof, it is possible that the spectra of the limited signal and of its periodic continuations overlap, even when the Nyquist criterion is met in the sampling for the original signal. In particular, the signal part of the first harmonic (0.14 sin 3xcfx890t in the above example) leads to audible distortions since this cannot be neglected compared to the fundamental. The following comments thus particularly relate to a reduction of the effects of the first harmonic but can also be expanded to harmonics of a higher order given the same procedure.
According to the invention, the sampling rate is raised given the digitalized signal with limited signal values (over-sampling). As a result thereof, the fundamental spectrum of this signal and its periodic continuations move far apart. The increase in the sampling rate occurs such that spectral components of disturbing harmonics no longer extend into the relevant range of the fundamental spectrum.
Particularly given an A/D-converted acoustic output signal of at least one microphone, the limitation of the signal values occurs to a maximum value in the case of the invention. This thus effects a change of the spectrum of the signal. The fundamental spectrum of the resulting signal is no longer band-limited; however, the signal strength drops with increasing frequency, so that the fundamental spectrum can also continue to be viewed as band-limited.
Advantageously, the boost of the sampling rate of the time-discrete, acoustic signal occurs following a digital signal processing. The digital signal processing in the hearing aid unit thus essentially occurs with the xe2x80x9cnormalxe2x80x9d clock frequency. This contributes such that the power consumption of the hearing aid having a level limitation of the invention is only insignificantly increased.
A system for boosting the sampling rate are known from the pertinent literature of the field, for example from Oppenheim, Schafer, xe2x80x9cZeitdiskrete Signalverarbeitungxe2x80x9d, Oldenburg 1992. A system for boosting the sampling rate of a time-discrete signal with a sampling rate expander as well as an interpolator proceed therefrom. The sampling rate expander effects an increase in the number of signal values of the time-discrete signal. The interpolator effects an approach of the signal having an increased number of signal values to the original signal curve. This approach can, for example, occur with a low-pass filtering. The approximation with a linear interpolator is in fact not as good but is simpler to realize. A number of other interpolators that can be utilized at this point are known in addition to the two that have been cited.
The fundamental spectrum of the resulting signal and its periodic continuations are respectively spaced from one another by the increased sampling frequency after the boost in the sampling rate has been carried out. The limitation of the digital signal values now occurs to a maximum value. Given a corresponding increase in the sampling frequency, disturbing spectral components now no longer occur in the fundamental spectrum.
According to a preferred version of the invention, the maximum value that limits the signal values is adjustable. The hearing aid can thus be set to the individual discomfort threshold of the hearing aid user. The setting can occur during manufacture, during fitting, or can be carried by the hearing aid user. One version of the invention provides separate level limitations in a number of channels of a multi-channel hearing aid. Such a hearing aid thus allows the setting of different discomfort thresholds for different frequencies. The hearing aid can thus be adapted better to the individual hearing of the hearing aid wearer.
In a further version of the invention, a signal post-processing unit is provided in the signal path following the unit for limiting the signal values to a maximum value. This, for example, can comprise a low-pass filter or a D/A converter. Due to the lower clock, however, the digital processing in a hearing aid should essentially occur in the signal processing unit.